The invention relates to a process for continuously printing a flat plastic film, flocked or not, the device for practicing the process and the printed plastic film obtained by this process.
The invention also relates to a process for continuously printing a colored or uncolored motif on a flat plastic film, flocked or not, adapted to be thermoformed to a predetermined non-planar surface, as well as the flat printed plastic film and the thermoformed plastic film obtained by this process.
Transfer printing by sublimation is already known as such and consists, in the first instance, in printing on a support paper designs from inks constituted by sublimable coloring agents which are adapted to vaporize at a certain temperature and to be fixed permanently on synthetic materials, for example polyester or polyamide polymers, the printing process of the support paper being adapted to be anything at all, for example offset printing, heliographic or flexographic printing, or again serigraphic printing by flat plate or rotary plate. Then, in the second instance, the designs in sublimable coloring agents, preliminarily imprinted on the support paper, are transferred by sublimation, by placing in contact, under a regulated pressure and at a predetermined temperature, generally with the aid of hot rolls, the pre-printed paper with the textile support to be printed, generally for a duration of 5 to 40 seconds. The hot rolls can comprise, in the case of printing in formats, a hot press with horizontal plates, or in the case of continuous printing from rolls of printed paper and of synthetic material to be printed, a rotating heated cylinder associated with a belt rolling under tension.
However, transfer printing by sublimation is not adapted to print plastic films, for the following reasons:
plastic films, and more particularly extruded films, have errors of flatness, which renders difficult their continuous printing, from a roll of plastic film, PA1 plastic films, and more particularly plastic films adapted to be thermoformed, such as polyvinyl chloride (PVC) films or polystyrene films, have a thermal sensitivity such that they melt and thus lose all cohesion, when they are subjected to impression by transfer by sublimation at a temperature of about 200.degree. C., which gives rise to random dimensional fluctuation of the plastic film during printing whilst the pre-impregnated impression paper undergoes no deformation, thereby giving rise to blurred and irregular printing, PA1 the melting of the plastic film during printing also gives rise to sticking of the plastic film on the conveyor belt of the printing machine, PA1 the melting of the non-flocked plastic film also leads to sticking of the plastic film to the pre-printed printing paper. PA1 applying on the back of the plastic film a temporary anti-adherent support, for example of paper, PA1 heating the assembly of the plastic film and of the temporary support to a temperature such that the plastic film reaches its softening point to stick temporarily to the temporary support, such that this latter ensures the dimensional stability of the plastic film during the following thermal printing step, PA1 transferring by sublimation a design constituted by a sublimable ink pre-printed on a printing paper, by contact under pressure and at a predetermined temperature with the front of the plastic film thermo-held on the temporary support, PA1 cooling the assembly of the plastic film thus printed and of the temporary support to a temperature permitting the unsticking of the temporary support. PA1 an unwinding station, comprising a reel for a strip of plastic film, flocked or not, and a reel for a strip of temporary support, PA1 a heating station at the inlet of which the strip of temporary support is applied against the back of the strip of plastic film, the assembly constituted by said superposed strips coming into contact via the temporary support strip, against a heating drum driven in rotation, so as to deliver at the outlet a composite strip constituted by the plastic film thermo-stuck to the temporary support, PA1 a thermo-printing machine, comprising a conveyor belt under tension which comes into contact with the temporary support of said composite strip, a heating calender driven in rotation, and at least one reel for a strip of paper pre-printed with a design constituted by a sublimable ink, said strip of pre-printed paper being applied to the front of the plastic film of said composite strip, upstream of the heating calender, such that the conveyor belt can press with predetermined pressure said composite strip covered with pre-printed paper against the heating calender, to transfer by sublimation the sublimable ink to the front of the plastic film, the printing paper being separated from the printed composite strip downstream of the heating calender, PA1 a cooling station to cool the composite printed strip to a temperature such that the temporary support strip can be unstuck from the strip of printed plastic film, PA1 a reeling station at the inlet of which the temporary support strip is unstuck from the strip of printed plastic film, each strip being wound on a respective wheel which is driven in rotation. PA1 geometrically dividing into elemental regions a portion of the flat plastic film to be thermoformed, PA1 determining for each elementary region of the film, the amount of local stretching of the film upon its thermoforming into the non-planar surface, PA1 determine for each elementary region of the film, a portion of the deformed design to be printed on the surface of the flat plastic film, this portion of the deformed design being defined by transformation by anamorphosis of the final design to be provided on the thermoformed non-planar surface, by taking account of the amount of local stretching of the plastic film by which one passes from the deformed portion of the design to the final portion of the design, PA1 printing on the surface of the portion to be thermoformed of the flat plastic film, the deformed design defined by the assembly of the portions of the deformed design determined by the preceding step. PA1 applying on the back of the flat plastic film a temporary anti-adherent support, PA1 heating the assembly of the flat plastic film and of the temporary support to a temperature such that the plastic film reaches its softening point to stick temporarily to the temporary support, such that this latter assures dimensional stability of the plastic film upon the succeeding thermal printing step, PA1 transferring by sublimation the pre-printed deformed design in reverse with a sublimable ink, onto a printing paper, by placing in contact under pressure and at a predetermined temperature said printing paper, with the front of the plastic film thermo-stuck to the temporary support, PA1 cooling the assembly of the flat plastic film thus printed and the temporary support to a temperature permitting unsticking the temporary support.
This printing process by transfer by sublimation can be used to print certain plastic materials, if there is first applied a layer of glaze on the plastic material to be printed, or first treating the pre-printed printing paper so as to render it non-adhesive. This printing method is particularly used to print pieces of plastic that are injected or shaped, such as bottles, skis, pens, helmets, and in this case, the printing is carried out by means of a heating tool matching the shape of the plastic piece to be printed.
Another direct printing method for plastic films consists in using known processes of the flexographic or serigraphic type, but in this case, the ink does not penetrate the plastic material, such that the printing designs have low resistance to scraping and friction. Moreover, these printing processes do not permit obtaining the most detailed graphic effects, for example for color printing of the photographic type.
Still another known method to print plastic films consists in thermobonding on the surface to be printed of the plastic film a pre-printed support, for example polyester film printed by heliography, then hot counter-adhered to the plastic film at the outlet of the extruder.
On the other hand, when plastic films to be printed are adapted to be thermoformed, for example to form protective blocks, pieces for the automobile industry, elements for packing, printed designs on the flat plastic film subject the deformation of the plastic film on a non-planar surface, giving rise to a modification of the contours and of the configuration of the printed design, as well as a variation of the shade of the colors, because of the stretching of the plastic film during its thermoforming. When the design which must be given to the non-flat thermoformed surface is a geometric design, letters or numerals, the deformations of the design can be unacceptable.